Why wont my 1990 Mazda B2600's engine start properly?

This sounds like a classic case of checking and rechecking everything you did to find out where the job went wrong. I’ll assume the engine was running before the spark-plug change and, if so, then it’s likely that you’ve done something really simple that has brought the vehicle to a halt.

The first thing to check is that you have replaced the spark-plug leads in their correct order. This is a really simple mistake to make but it will definitely put an end to play if you get it wrong. Even experienced wise heads often tackle this job one spark plug and one lead at a time to avoid a tangle of leads and the ensuing mix up. While you’re there, check that the high-tension lead from the coil to the distributor cap hasn’t fallen out or become loose. Sometimes it will look as though it’s in place, but needs a push down on to its contact. Still at the coil end of things, check that none of the little spade terminals have been accidentally dislodged from the coil’s terminals.

Another common mistake is to simply buy new spark plugs and install them. In many cases, they need to be gapped correctly, that is, they need to have the gap between the two electrodes set at the correct distance for your engine. In the case of your car, the correct gap is 1.1mm (44 thou, for the old mechanics out there). Without the correct gap, the engine won’t run properly and, if the gap is out far enough, the engine may not run at all. Don’t assume the spark plugs will have the correct gap on the shelf at the spare part shop.

All this pre-supposes, of course, that the problem has occurred as part of the spark-plug change. The truth is that it could also be a coincidence where something with the fuel system has gone wrong at the same time as you decided to change the plugs. If that’s the case, it’s back to the drawing board in terms of a diagnosis, but the good news is that the engine in your car is quite a simple one and is a great engine on which to learn about mechanics. Above all else, don’t be discouraged and keep at it; home maintenance of a vehicle can be very rewarding and save lots of money.